1. Field of the Invention
The present invention relates to an image sensor and a method of fabricating the same.
2. Description of the Prior Art
A light receiving element of a sandwich type which uses as its photoconductive layer an amorphous semiconductor layer made of such material as hydrogenated amorphous silicon (a-Si:H) or a polycrystalline thin film made of such material as cadmium sulfide (CdS) or cadmium selenide (CdSe) exhibits an excellent photoelectric conversion characteristic, is simple in structure and easily made to provide a large light reception area. For these reasons, such elements have found wide applications to original document readers which comprises a contact type image sensor based on elongated reading elements having the same sensor section width as that of original documents. That is, the contact type image sensor is a large-area device not requiring any reduction optical system when reading documents.
However, the capacitance storage type image sensor is defective in that as the more integrated and elongated sensor is demanded, variations in electrostatic capacities between lines of a wiring section resulting from different distances of the respective light receiving elements from their sensor regions to a drive section (IC) becomes unnegligible, because such capacitance variations result in an irregular output signal of the sensor.
The sensor section of the image sensor is basically arranged so that, as shown in FIG. 18 (a) and (b) (FIG. 18 (b) is a cross-sectional view taken along line A--A in FIG. 18 (a)), a photoconductive layer 4 is sandwiched by a light permeable upper electrode 3 and a plurality of lower electrodes 2 arranged in a row on a substrate 1, which is covered with a light-shielding film c having open window a. The light receiving elements form respectively such an equivalent parallel circuit of a photodiode 5a and a capacitance 5b as shown in FIG. 19. In the contact type image sensor, a necessary number of such light receiving elements 5 are arranged on the elongated substrate in its main scanning direction at a density (for example, 16 elements/mm) necessary to resolve original document and are electrically connected through their wiring sections 6 to a drive section D. The section D comprises a power source 7, a shift register 8, and MOS field effect transistors 9 (MOSFET's) connected to the sensor sections of the light receiving elements so that driving of the shift register causes sequential switching between the sensor sections and the power source 7. When first driving of the shift register 8 causes the MOSFET's to be sequentially turned ON and OFF, a sequential closed loop is completed between the power source 7 and the respective sensor sections of the light receiving elements 5 so that electric charges resulting from the loop completion are stored in the capacitor 5b of the sensor section of the associated light receiving element itself and in a capacitor 6b of the associated wiring section 6. The stored charges are neutralized by light incident upon the associated sensor section or remain. Thereafter, second driving of the shift register 8 causes the MOSFET's to be sequentially turned ON and OFF again so that recharging of the associated capacitors 5b and 6b causes a current to flow through a signal line 10 by an amount corresponding to the remained charges in the capacitors 5b and 6b of the associated bit. Such current is outputted for every bit as a read output of the contact type image sensor. In this manner, the above-mentioned operation is repeated for every line for reading operation of the original document.
On the other hand, the wiring sections are usually formed on the same substrate 1 as the sensor sections for interconnection between the drive section D and the sensor regions. However, the sensor regions must be connected to the shift register or MOSFET by wire bonding or the like, which inevitably causes different lengths of wiring sections 6 and thus the electrostatic capacities Cx of the capacitors 6b formed by the wiring sections 6 vary.
Now, the circuit of FIG. 19 will be studied. Assuming that the capacitor 5b has an elctrostatic capacitance of Cs, the capacitor 5b is initially charged with V (a) (V (o)=V.sub.B), a photocurrent Ip flows through the photodiode 5a and S is a Laplace operator. When attention is directed only to one of the light receiving elements in the circuit of FIG. 19, the equivalent circuit of the one element is as shown in FIG. 20 (a). The circuit of FIG. 20 is converted into an equivalent circuit of FIG. 20 (b) when the element is subjected to no light and into an equivalent circuit of FIG. 20 (c) when the element is subjected to light.
V.sub.B /S in FIG. 20 (b) and (c) satisfies the following relations (1) and (2) respectively. EQU V.sub.B /S=I.sub.B {(1/SCs)+(1/Scx)} (1) EQU V.sub.B /S=I.sub.A {1/SCx}+{I.sub.A -(I.sub.p /S)}/SCs (2)
Further, Vout(s) is given as follows. EQU Vout(S)=(I.sub.A -I.sub.B).multidot.(1/SCx) (3)
The equations (1) and (2) are rewritten as equations (4) and (5), respectively. EQU I.sub.B ={CsCx/(Cs+Cx}V.sub.B ( 4) EQU I.sub.A ={1/(Cs+Cx)}.multidot.{(CsCxV.sub.B)+(CxI.sub.p /S.sup.2)}(5)
Accordingly, Vout(S) is rewritten as follows in accordance with the equations (4) and (5). ##EQU1## Hence, EQU Vout(t)={I.sub.p /(Cs+Cx)}t (6)
As will be seen from the equation (6), the read output of each bit (element) of the contact type image sensor depends on a total value of the capacitance Cs of the capacitor 5b formed by the sensor region itself and the capacitance Cx of the capacitor 6b formed by such an attachment circuit as the wiring section. In this way, the prior art image sensor has a problem in that the sensor has an irregular read output.